The invention relates to a process for the production of dyed, homogeneous flat articles based on thermoplastic synthetic resins wherein particles of thermoplastic synthetic resins are sintered into a coherent flat article, such as a sheet and the article is then press-molded under the action heat and pressure and wherein the starting material for forming the particles is obtained from resin preheated above the softness temperature of the resin.
Unicolored, homogeneous flat articles made of a thermoplastic synthetic resin, especially made of polyvinylchloride (PVC), in the form of sheets or panels, with thicknesses of above 1 mm, can be manufactured according to the state of the art by means of extrusion, spread-coating, or rolling methods, resulting in a unicolored, homogeneous molded article conveying, as an inherent design feature, a cold plastic look and a typical plastic image. Even mixtures of various granulated compositions and their processing by means of the extrusion or rolling technique, though providing a color differentiation corresponding to the colors utilized, have not contributed toward lifting the cold feeling inherent in plastic. However, especially when using sheets or panels as floor coverings and wall coverings, special emphasis is placed on esthetic impression. In this connection, it is an object of the invention to create flat articles of thermoplastic synthetic resins conveying an esthetically pleasing optical impression and lending themselves optionally to random patterning.
A process has been known from U.S. Pat. No. 3,381,067 for the manufacture of terrazzo-type floor coverings wherein qranules having a diameter of 6-12 mm and a thickness of 44-800 .mu.m are produced from a plasticized PVC with fillers and colorants and stabilizers, and coated with a -plasticized PVC paste, then piled up into a uniform layer, heated, sintered, and press-molded into a continuous sheet with the use of pressure and heat. By utilizing different compositions for the granules and the coating step, a covering is produced wherein the granules are embedded in a softer shell, i.e. a covering exhibiting differing wear characteristics.
U.S. Pat. No. 3,192,294 discloses a process of the type for the production of floor coverings having a multicolor effect, for example pepper-and-salt patterns, wherein preferably compositions containing PVC, plasticizer and blowing agent in powder form are piled up into a layer and heated to such a degree that the layer is sintered into a coherent sheet which is then expanded by decomposition of the blowing agent, whereafter the sheet, while passing through a roll nip, is embossed on the topside with a relief occupying, e.g., one-half or more of the sheet thickness. The PVC composition is prepared as a dry blend, i.e. mixed in the cold state (also in order to avoid premature decomposition of the blowing agent), and the particles exhibit an average size of 15 .mu.m to about 3 mm, using in each case a uniform particle size, for example, of 15 .mu.m. It is also possible to furthermore add proportions of very small particles of 2 .mu.m made of a copolymer, but this can be very disadvantageous if the fine proportions are separated, i.e. fall through toward the bottom, and the coarse particles form an uneven surface.
The floor coverings produced in accordance with U.S. Pat. No. 3,192,294 with thicknesses of about 2.5 mm are prone to cracking on account of their strong surface embossing, and thus require additional surface protection in the form of an additional film coating. Furthermore, the particle structure is shifted due to the vigorous embossing, so that elongated, nonuniform zones are formed.
U.S. Pat. No. 3,359,352 and German Pat. No. 1,298,023 describe a process for the production of a decorative, multiple-layer covering wherein particles having a grain size of about 2.5 mm to 0.013 mm, produced by mixing PVC powder and plasticizer as well as additives at temperatures of about 93.degree. C., i.e. well below the sintering temperature of the synthetic resin, are piled up on a substrate sheet serving as the support to form a layer and are sintered into a porous sheet. These particles, produced at low temperature far below the plasticizing temperature of the synthetic resin, also called dry blends, are soft and do not show high firmness and dimensional rigidity since they have not been plasticized. During the subsequent press-molding step, the particles thus deliquesce and, when using differently colored particles, result in mixed colors or in deliquescing color contours with an orientation in the rolling direction. It is necessary in this conventional method to utilize a support material as the substrate layer homogeneous flat articles, i.e. layers showing a design throughout without a substrate layer, consequently cannot be manufactured.